Included in concentrated-solar technologies, are central tower receiver systems where direct solar radiation is reflected by a fields of heliostats pointed towards a receiver. The receiver is the component where all the solar radiation is concentrated and through which the energy is transferred to a working fluid that can reach temperatures beyond 1000° C.; this energy is then used to generate electricity.
There are currently various types of receivers available with different working fluids and different ways in which energy is transferred to the latter and varying receiver configurations too. Also, there are receiver tubes, volumetric receivers, receivers that exchange energy directly or indirectly, saturated steam and superheated steam receivers, among others.
The concentrated solar tower receivers can be external or they may have a cavity located in the upper part of the tower with the aim of reducing heat loss. The configuration must enable the incidental power to exceed the magnitude of the loss through radiation and convection. With superheated steam receivers, the temperature reached on the surface is greater than with the water/steam ones, which is why the losses through radiation are also greater, however, they have the advantage of increasing the efficiency of the thermodynamic cycle, and therefore the losses are offset.
So the main advantage of superheated steam receivers is that by working with a more energetic fluid, they increase the turbine efficiency and that of the thermodynamic cycle, thereby reducing the costs of producing electricity. It is estimated that the efficiency of the cycle can increase by 10% and the electric energy production could reach 20%.
The idea of using superheated steam receivers in concentrated solar tower receivers was implemented in Cesa-1 and Solar One projects in the 80s. The Cesa-1 project, situated in the Almeria Solar Platform, had a receiver cavity composed of an evaporator and two super heater beams on top of the evaporator. The Solar One receiver had an open cylindrical structure that was easier to build than the Cesa-1 receiver, but with greater heat loss.
There were technical inconveniences in operating the plants for both projects related mainly to the resistance of the materials and controlling the systems under transitory conditions. Cracks appeared in the upper part of the receiver's subpanels in Solar One, caused by the difference in temperature between one panel and another, which caused the working fluids to leak; the proposed solution was to decrease the temperature gradient between the panels with some structural changes. In Cesa-1, the problems experienced were caused by the thermal inertia of the system that caused flooding in the super-heater collectors.
In view of the foregoing, the superheated steam receivers can suffer damage to their structure due to the high operating temperatures, the distribution of incidental fluids (not uniform) and the thermal pressure to which the material is subjected. The thermal cycles are generated by surface environmental exposure, radiation reflected by the heliostats (reaching temperatures of nearly 600° C.) and the temperature gradient of the working fluid between the entry and the exit of the component (the steam enters at around 250-310° C. and exits at 540° C.).
The aforementioned inconveniences with the superheated steam receivers can be reduced by eliminating the coexistence of the liquid-steam phases inside the tubes and with the suitable configuration of the elements that make up the solar component. This is where the importance of the design and configuration of the receiver takes effect, enabling the correct operation and control of the system and guaranteeing the integrity and durability of the structure.